llvm dev - Dec 2018 - Command line -mcpu= and -march=

Hi

I'm doing a port of Clang/LLVM - purely for the educational value - to the
MC6809 (and HD6309 as a dub target) processors. I'll also want to eventually
provide direct support for the AM9511 floating-point coprocessor as an option.
I'm working with bleeding-edge code from the Git mirror.

I've made some pretty decent progress; I copied the llvm/lib/Targets/MSP430
Target to llvm/lib/targets/MC6809, changed all references to MSP430 to MC6809
ones, and went through the rest of the code mechanically creating MC6809 entries
wherever there was an MSP430 one. Similarly in llvm/tools/clang/...

I then butchered the MC6809.td file, the register description .td file, made a
new machine description string, and fixed the compilation problems that arose.
This gives me a FrankenComopiler that generates MSP430 instructions using MC6809
registers in MC6809 memory layout. So far so good. I'm leaving the fun bit -
doing all the instructions/assembly/output file generation - for last.

What I can't figure out is how to the -mcpu=foo and/or -march=bar to work.
The documentation implies that you get these for free by setting up the
processor and subtarget features in the MC6809.td file, but try as I might I
can't get it to work, and it doesn't work for the MSP430 either.

No matter what I do,

$ clang --target=mc6809-elf -march=foo -S ~/test.c -o test1.s

... gets me a response of

clang-8: warning: argument unused during compilation: '-march=foo'
[-Wunused-command-line-argument]

... even for MSP430 ...

$ clang --target=msp430-elf -march=baz -S ~/test.c -o test1.s
clang-8: warning: argument unused during compilation: '-march=baz'
[-Wunused-command-line-argument]

I'm concluding that I need to write a "consumer" of the -march
and/or -mcpu options myself, but I can't find a canonical example of this
anywhere. I've played with RegisterTarget, but the documentation is a bit
too densely packed for me. Other CPUs like ARM and AArch64 work, and I can't
figure out why.

Can anyone please point me at how to canonically add a -march= or -mcpu= option
to MSP430? I can take it from there.

Thank you!

M
-- 
Mark R V Murray

Hi Mark,

On Wed, 19 Dec 2018 at 10:23, Mark R V Murray via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> What I can't figure out is how to the -mcpu=foo and/or -march=bar to
work. The documentation implies that you get these for free by setting up the
processor and subtarget features in the MC6809.td file, but try as I might I
can't get it to work, and it doesn't work for the MSP430 either.
This probably applies to the developer tools (llc, opt etc), but Clang
needs a bit of attention since it often tries to present a sanitized
view of those internals to end users.
> ... even for MSP430 ...
MSP430 is a fairly bare-bones target, I'm not entirely surprised
no-one has done the work there.
> I'm concluding that I need to write a "consumer" of the
-march and/or -mcpu options myself, but I can't find a canonical example of
this anywhere. I've played with RegisterTarget, but the documentation is a
bit too densely packed for me. Other CPUs like ARM and AArch64 work, and I
can't figure out why.
I think the top-level code you want is in (under clang)
lib/Driver/ToolChains/Clang.cpp (a bunch of getTargetFeature stuff)
and lib/Driver/ToolChains/CommonArgs.cpp (getCPUName especially).
These defer to target-specific .cpp files in the more complete targets
(lib/Driver/ToolChains/Arch/XYZ.cpp).

Cheers.

Tim.

> On 19 Dec 2018, at 10:34, Tim Northover <t.p.northover at gmail.com>
wrote:
> 
> Hi Mark,
Hi Tim,
> I think the top-level code you want is in (under clang)
> lib/Driver/ToolChains/Clang.cpp (a bunch of getTargetFeature stuff)
> and lib/Driver/ToolChains/CommonArgs.cpp (getCPUName especially).
> These defer to target-specific .cpp files in the more complete targets
> (lib/Driver/ToolChains/Arch/XYZ.cpp).
Spot on! Thank you!

I got it working now :-)

M
-- 
Mark R V Murray